1. Field of the Invention
The present invention relates to magnetoresistive (MR) sensing devices, and more particularly, to MR sensing devices for measuring magnetic fields having improved rejection of stray magnetic fields, and temperature stability, and other improved features. Such devices find utility as current sensors and diagnostic devices in motor controllers, as sensors for powerline communication systems, for position sensing using the fields of permanent magnets to indicate position, and in other applications where information can be derived from time or spatially varying magnetic fields.
2. Related Art
Conventionally, current flowing in a circuit has been measured using current transformers or Hall Effect sensors or by measuring voltage across a reference element in the circuit. Such current sensors are subject to several disadvantages, especially when used with motor controllers.
For example, current transformers are frequency sensitive, and in any event, occupy substantial space, while Hall Effect devices requires use of toroids which exhibit hysteresis and limited bandwidth. Measuring voltage across a reference element is also not completely satisfactory due to insertion loss in the sensing element, problems with signal couplers, etc.
Certain nickel-iron alloys such as Permalloy (Ni81Fe19) are known to be magnetoresistive, i.e., to exhibit electrical resistance which depends on the strength and direction of nearby magnetic fields, and it has been proposed to use such devices as current sensors by combining several sensor elements in a Wheatstone bridge mounted in proximity to a current-carrying bus or a current trace on a semiconductor substrate. A reference voltage is applied to the bridge and voltage output, which changes due to magnetically induced resistance changes, is measured as an indication of an incident magnetic field.
However, known MR sensing elements exhibit some undesirable properties. Among these are excessive responsiveness to stray magnetic and electric fields, narrow range of linearity (making measurement over a large current range difficult), sensitivity to changes in ambient temperature, (with the consequent need for careful calibration during installation, and frequent re-calibration depending on the accuracy required). Various techniques have been proposed for dealing with these problems, but these are often complex and costly, and no completely successful solutions are known to exist.
Further, to avoid the need to insert a bus section into the current carrying circuit, some existing devices have employed multiple sensor chips installed in mounting clamps which are attached to the current carrying busses. The valve of this approach, however, is also problematic as such devices are quite expensive and the mounting clamps makes them inconvenient to use, especially for measurements made on printed circuit boards. Another approach has multiple sensor chips arranged around a slot or hole through which passes the current carrying conductor or above/below or on either side of a circuit board trace or conductor mounted on the circuit board. This approach allows for PC board mounting, but requires two sensor chips and takes up extra space.
Thus, a need clearly exists for improved MR sensors for measuring current in motor control devices and similar applications which overcome the above problems. The present invention seeks to meet this need.